I see that Ext JS is already using the 'caching' technique in the context of the Object.prototype.toString method, but the Object.prototype.hasOwnProperty method is left out.

Keep in mind that Ext JS has fairly long prototype chains, so it would be a good idea to save a reference to core methods such as hasOwnProperty. I checked some other JS libraries, and they use this technique.

Of course, this won't provide a knockout performance gain, but such small improvements will add up and are worth considering.

1) The wasn't a consistent performance improvement across browsers. It was better in most, but not all.
2) The amount of improvement was a fraction of a percent, it's a micro optimization that's likely to have little or no noticeable impact.